Increasingly widespread utilization of forest resources has led to an increased scarcity of old-growth timber in many parts of the world. This is unfortunate because older trees generally have denser greater percentage of "mature" wood (per unit volume of wood) in older trees relative to "juvenile" wood. Lumber having a greater percentage of mature wood tends to be stronger than lumber having a greater percentage of juvenile wood. This is one reason why old-growth lumber is in such high demand.
As the supply of old-growth timber decreases, more and more lumber is being obtained from trees grown on plantations and similar types of environments in which the trees are urged to reach harvestable size as soon as possible. Because such trees grow so rapidly, the percentage of juvenile wood in them is high relative to mature wood.
Although wood used for construction has any of various standardized width and thickness dimensions, lumber having a higher percentage of juvenile wood is generally structurally weaker than similarly sized lumber having a higher percentage of mature wood. Thus, certain plantation-grown lumber having a high percentage of Juvenile wood is simply not strong enough for certain uses or to comply with construction codes. Of course, it is possible to increase the cross-sectional profile of such lumber to increase strength and stiffness. However, such a remedy is not practical in some instances and not cost-effective in many instances. Thus, as increasing amounts of lumber are produced in plantations and other secondary-growth sources, there is substantial interest in providing ways to enhance the strength and stiffness of such lumber without increasing the physical dimensions of the lumber.
Also, as demand increases for the available supply of timber, a correspondingly greater need arises to exploit all sources of wood and wood fiber. For example, materials previously considered to be "waste," such as sawdust, wood chips, and the like, are now being utilized for making various types of particle board, chip board, laminates, and other useful products. These products are often termed "enhanced" wood products because their composite nature tends to augment certain physical properties compared to ordinary lumber.
One of the earliest such enhanced wood products is a material termed "compreg," described in Report No. 1381 from the Forest Products Laboratory, U.S. Dept. of Agriculture, Forest Service, Madison, Wis. (1955) entitled, "Resin-Treated, Laminated, Compressed Wood (Compreg)". Compreg is made by saturating a wood (such as a wood veneer) with a phenol-formaldehyde resin, then compressing the fully impregnated wood under elevated temperature. The resulting product exhibits certain enhanced physical properties such as improved resistance to moisture absorption, increased hardness, and greater dimensional stability compared to conventional wood. Unfortunately, the amount of resin necessary to saturate the wood renders compreg not economical and/or impractical for many uses, particularly for construction uses.
A number of other methods for enhancing the strength of wood are known in the prior art. However, the products made by such methods have drawbacks that render them unsuitable for certain uses.
Thus, there is a need for ways to modify less dense wood so as to confer on the wood an ability to structurally perform in a manner similar to high-quality old-growth timber, particularly in instances in which the physical dimensions of the lumber, such as thickness, cannot be increased to provide greater mechanical strength or stiffness.